The present invention relates to adhesive compositions and to composite materials made from such adhesive compositions and solid foam materials. In further aspects it relates to methods of making such composite materials, and articles made from them. The invention is primarily concerned with composite materials which are porous, resiliently compressible, and water resistant. Such materials may be particularly suitable for the manufacture of insoles and handgrips (e.g. for sports rackets).
The most widely used solid foam materials are polyurethane foams. As is well known, these can be produced with a wide range of rigidity (from very soft to rigid) and a wide range of density. A further variable is whether the material is open-cell (and hence porous) or closed cell.
Low density flexible polyurethane foams are materials of density 10-80 kg/m3, composed of lightly cross-linked open cells. Air can flow easily through them. They are soft, with little strength or resistance to abrasion. Their net-like surface layers make it difficult to bond them to other articles, e.g. for lamination. They are typically used as cushioning, in bedding or upholstery. In contrast, polyurethane foam materials conventionally used for shoe soling and the like are microcellular foams (i.e. denser than xe2x80x9chigh density foamsxe2x80x9d), with intermediate softness. Such materials have little if any porosity. They are generally self-skinning, which further reduces any porosity they may have. Thus they typically have to be needled if they are required to be breathable.
In one aspect the present invention enables one to reinforce a low density flexible foam while preserving substantial porosity. Use is make of a water-resistant, water-transmissive glue. This may be based on a glue which is not substantially water-transmissive, typically a latex-type adhesive, in which a water-absorbent filler has been incorporated so that a film formed from the adhesive is water absorbent.
It is known to incorporate fillers, including water-absorbent fillers, in adhesive compositions. However they are incorporated at low levels (e.g. xc2xc-1% w/w) and do not confer water-absorbance.
Thus in a first aspect the invention provides a composite material formed from a low density flexible plastics foam material (preferably 10-80 kg/m3) and an adhesive composition comprising a water-resistant glue composition and a water-absorbent filler, the filler being incorporated in an amount sufficient to confer water absorbance on a film formed from the adhesive composition. The foam material will generally be porous, and the composite material desirably retains substantial porosity.
Generally the glue composition is a latex e.g. a styrene-butadiene-rubber (xe2x80x9cSBRxe2x80x9d) latex (preferably carboxylated) or a dispersion based on polyurethane or acrylonitrile-butadiene or acrylics or ethylene-vinyl acetate (EVA) or copolymers or terpolymers based on any of these. Generally it will be settable or curable to a non-tacky state. That is, it is not a pressure-sensitive adhesive as used on xe2x80x98self-adhesivexe2x80x99 articles such as labels and tapes. Neither is it a remoist adhesive (as used on stamps etc.)
The glass transition temperature of the glue composition may be 10-15xc2x0 (or lower if intended for low temperature use, e.g. 0xc2x0 or less).
The amount of filler is preferably 5-7% of the adhesive compositions (w/w). Suitable fillers are inorganic materials of small particle size (e.g.  less than 5 xcexcm, preferably  less than 1 xcexcm.). Silica, particularly fumed silica, is currently preferred. Calcium carbonate is also usable.
The adhesive composition may be formed by mixing components A and B in approximately equal amounts by weight (e.g. 40-60% of A, 60-40% of B), where:
Component A is a latex of solid content 40-55% by weight (preferably a carboxylated SBR latex); and
Component B comprises water (86-90 parts) and filler (preferably silica such as Aerosil 200) (10-14 parts). It may further include a bactericide (1-2 parts). (All parts are by weight.)
The compounded adhesive may also include a crosslinker which is reactive with carboxy and hydroxy groups, e.g. a polyfunctional aziridine such as trimethylolpropane-tris-(xcex2-(N-aziridinyl) propionate (available as XAMA-2 from EIT Inc), or CX-100 (Zeneca Resins). Use of 0.5-2% w/w of such a trifunctional linker can improve the water- and chemical-resistance and bond strength of the adhesive.
Alternative adhesive compositions which may be used are breathable adhesives or coatings. Their xe2x80x98breathabilityxe2x80x99 may be due to their molecular structure and is generally not dependent on fillers. Known examples include water-based polyurethane products such as Witcoflex 120 and 130 (Baxenden Chemicals). These provide water-resistant, breathable, hydrophilic coatings. However they are relatively expensive. Furthermore they tend to be too soft for various applications such as insoles.
The plastics foam material is preferably a polyurethane foam. (Other possibilities included polyethylene.) Its density is preferably 10-60 kg/m3, more preferably 20-60, still more preferably 30-50 and most preferably 40-45 kg/m3. It is desirably a flexible, fully open, sponge-like material. It is generally 3-12 mm thick. For use in producing insoles for normal use, thicknesses in the range 4-6 mm are preferred. Insoles for sports shoes may be up to 10 mm or more. Material for handgrips may be 3 mm thick. (Thinner foam is not readily available.)
The adhesive is preferably a fillerxe2x80x94containing adhesive as described above or, less preferably, a breathable adhesive.
In a second aspect the invention provides an adhesive composition comprising a water resistant glue composition which is a latex or other aqueous polymer dispersion and, as a filler, fumed silica incorporated in an amount such that a film formed from the adhesive composition is water absorbent (said composition setting or curing to a non-tacky state).
In a third aspect the invention provides an insole comprising a composite material according to the first aspect.
In a fourth aspect the invention provides a handgrip (e.g. for a sports racket or hand tool) comprising a composite material according to the first aspect.
In further aspects the invention provides methods of producing composite materials according to the first aspect or articles according to the third or fourth aspect; and a method of producing or repairing a handgrip, optionally in situ on a handle. Application of a coating of an adhesive composition as used in the present invention to a foam-based handgrip, or a foam substrate for use in forming a handgrip, leads to a handgrip of enhanced durability. The filler in the adhesive composition can provide durability and good nonslip properties, as well as ensuring that the set adhesive is able to absorb moisture.
In one type of embodiment, the adhesive composition is primarily present in a fabric layer which is adhered thereby to a major surface of the foam material. Preferably the foam material is in the form of a sheet, and said fabric layers are adhered to both major surfaces. This gives a product which is reversible and washable. Preferably the fabric was soaked in the adhesive composition and the setting thereof both adhered the fabric to the foam material and provided the fabric with a protective coat. The fabric is preferably of a water-absorbent material, e.g. based on cotton (e.g. pure cotton or cotton-polyester or other known blend).
The fabric may be woven (cloth) or nonwoven. Suitable nonwovens include highly porous materials, composed of fibres of synthetic polymer materials.
In a second type of embodiment, the plastics foam material was saturated with the adhesive composition which was subsequently cured to produce a reinforced foam material. This type of embodiment generally does not have the cloth surface layer(s). The plastics foam is preferably of thickness 4 or 5-12 mm.
A preferred method of producing the first type of embodiment comprises the following steps: (a) fabric is soaked with the adhesive composition (suitably by immersion); (b) the soaked fabric is applied to the foam material and the assembly is oriented so that the fabric is on the underside; and then (c) heat is applied to the fabric, preferably using a jet of hot gas or infra red irradiation. The orientation of the assembly reduces the penetration of the adhesive composition into the foam material.
For insoles, the foam is typically about 5 mm thick and is preferably 30-40 kg/m3. It may be reinforced either by the adhesion of a fabric layer to one or both faces, or by the saturation method referred to above.
For handgrips, a foam layer about 3 mm thick is preferred, and reinforcement is preferably by adhesion of a fabric layer. For providing a protective coating on a handgrip, it is preferred to use an adhesive composition according to this invention, particularly one based on a polyurethane dispersion with fumed silica as filler.
The following table compares the properties of various materials for forming insoles. xe2x80x9cPresent inventionxe2x80x9d refers to preferred embodiments of the present invention of the type consisting of polyurethane foam with cloth bonded to both faces. The adhesive used is based on a carboxy SBR latex containing silica. The cloth was saturated with the latex composition which thus provided surface protection for the cloth as well as bonding it to the foam.